The present invention relates to measurements of light-transmissive thin films or layers of such films, and to measurements of surface topography or detection of surface defects. It specifically relates to optical measurement or detection, and to apparatus for performing such optical measurement or detection of a thin film.
A number of common articles of manufacture now have constructions involving thin films formed on relatively large area smooth substrates, and substrates wherein the underlying surface is reflective, possibly conductive, and at least visually smooth if not optically flat. To develop manufacturing processes for reliably fabricating these articles and to inspect them or understand the defects which arise in these articles, it is necessary to observe the thin films. These films may be liquid or solid, have a thickness substantially under one wavelength of the observation illumination, and may possess features or defects which are observable only with meticulous methodology against the highly reflective substrate, requiring a special probe. To detect changes occurring on such a thin surface coating is an even more challenging task.
In part, sensitivity of a probe is determined by devising a standard against which its measurements can be compared. Thus, for example, U.S. Pat. No. 4,873,430 describes a technique for measuring a thin film or surface effect in which all reflected light is collected by a nearly-contacting detector, and in which a certain degree of signal normalization is achieved using separate detectors for specular and diffuse light collection. A similar device has also been described in a paper of Meeks, Weresin and Rosen presented at the ASM/STLE Tribology Conference, Maui, Hi. in Oct. 15-19, 1994. That system employs an integrating sphere substantially contiguous with and tangent to a disk to capture all diffuse reflection from the disk surface; apertures through the sphere allow two light paths to be directed at the tangent point for illumination of the point at 60.degree. incidence and collection of the specular component reflected from that point. The device includes automated scanning and collection, and autocalibration is achieved using reference mirrors.
While each of these approaches utilizes sound theoretical models to address the problem of obtaining normalizable and meaningful levels of signal from a thin film, many practical realities should be recognized in the operation of these devices. Thus, for example, the taking of reflectance measurements on a micrometer-scale region may require multiply-repeated sampling and averaging, or integrating, to obtain a repeatable result. Furthermore, while it may be considerably more informative to compare like measurements taken from many consecutive points on a single disk, even the collection of many thousands of data samples may still provide relatively sparse information about the total working surface of the disk. Furthermore, one of the more interesting avenues of research, involving wear test measurements taken from the same disk before and after use, is severely hampered by the difficulty or impossibility of achieving submicron registration of the disk with an earlier position once it has been removed from the measurement stage. This restricts such tests to the brief wear testing that can be performed while the disk remains on the optical measurement stage, or to relatively uninformative comparison of measurement ensembles, rather than pointwise measurement comparisons taken on a single disk over an extended time.
Accordingly, improvements in optical film measurement apparatus are desirable.